Daylilies (Hemerocallis fulva) are one of the most common bulb plants in America. These showy flowers are easy to grow, extremely hardy and make terrific cut flowers, as they can last a long time in a vase. Some of the over 40,000 cultivators also have a wonderful fragrance. Daylililes, however, are not true bulb plants. They have extremely thick tubers which send up new sprouts. They also reproduce by seed, but it's the tubers that cause rapid spreading. The tubers tend to grow into each other, forming large, hard clumps of root masses that can be almost impossible to separate.
Choose locations that make it difficult for daylilies to spread. These hardy flowers will grow just about anywhere, but if they get full sun and well-drained soil they will spread more rapidly than you would believe. Instead, plant them in a shady location, such as under a tree where the tubers will have to compete with thick tree roots.
Separate them frequently. Use a shovel to dig up the tubers every 2 years in the fall and break them apart with your hands or with the shovel. Make sure there is a sprout on each section of tuber. Then, replant them as new plants or give them away. Separating them frequently will ensure that they do not grow together.
Place a border around your daylilies. There are many flexible borders you can buy that are made to sink into the ground and surround the plant, thus preventing the horizontal spread of the tubers. Most are marketed to contain bamboo plants. Dig a trench about 6 inches around your flowers, and insert the border, overlapping the ends. Fill it in with soil. Note that the daylilies will continue to spread within the bordered area, however.